1. Field
The disclosed technology relates to electronic systems and, in particular, to variable frequency circuits.
2. Description of the Related Art
Certain electronic systems, such as base stations, can include an attenuator to reduce an amplitude and/or power of a radio frequency (RF) signal without appreciably distorting features of the RF signal. Some attenuators can include an amplifier, a digital step attenuator (DSA), and a serial peripheral interface (SPI) controller. In some implementations, the SPI controller can convert a serial control word into an output word, which can control switches in the DSA. The bit stream can represent a time series of one or more bits. The output word can be represented as differential signals to increase performance and/or reduce power consumption. A negative voltage level can be used to generate the differential signals, for example, using a level shifter. The negative voltage can be generated by the SPI controller. For example, an oscillator can generate an output having a fixed frequency, and provide the output to a charge pump via a buffer. The charge pump can then generate the negative voltage.
Electronic systems that include attenuators can have stringent noise specifications that are difficult to meet. Conventional negative voltage generators can introduce noise in the form of spurs, which may also be referred to as “spurious tones,” at harmonics of a frequency of an oscillator. Such noise can be introduced into RF inputs and/or RF outputs of some electronic systems. This can, for example, reduce noise margins. Accordingly, a need exists for reducing spurious tones in electronic systems, such as electronic systems that include attenuators.